The present invention relates to a transmitter for a wireless tire condition monitoring apparatus and to the tire condition monitoring apparatus that allow a driver in a vehicle passenger compartment to check the conditions of vehicle tires, such as air pressure of the tires.
Wireless tire condition monitoring apparatuses for allowing a driver in a vehicle passenger compartment to check the conditions of vehicle tires have been proposed. One such monitoring apparatus includes transmitters and a receiver. Each transmitter is located in one of the wheels and the receiver is located in the vehicle body of the vehicle. Each transmitter detects the conditions such as air pressure and the temperature of the associated tire by, for example, a pressure sensor and a temperature sensor and wirelessly transmits the detection data to the receiver using carrier waves of a predetermined frequency. Thus, each transmitter has a surface acoustic wave (SAW) resonator for generating the carrier waves. The receiver receives the data from the transmitters via a reception antenna and displays the condition of each tire on an indicator located, for example, in front of the driver's seat.
The SAW resonator generally has a frequency deviation of ±300 ppm due to manufacturing tolerance. Therefore, the transmission band of the transmitter tends to vary due to the frequency deviation of the SAW resonator. Accordingly, the reception band of the receiver needs to be widened. However, the receiver having a wide reception band has an increased possibility of receiving signals other than reception signals. Further, the receiving sensitivity could decrease due to deterioration of the signal versus noise ratio. As a result, the transmission data from the transmitter is not easily received accurately.
To solve the above problem, a conventional transmitter uses a high precision SAW resonator having the frequency deviation of ±50 ppm or less. However, the high precision SAW resonator is expensive, which increases the cost of the transmitter.